Thin film deposition methods are commonly used for the fabrication of semiconductor and other electrical, magnetic, and optical devices. However, the quality (material properties) of thin films deposited by conventional methods are often not comparable to bulk material, particularly in cases of low temperature deposition, such as when temperatures at the substrate must be kept much lower than the melting point of the films to avoid thermal damage to the devices. This is often a result of imperfections in the as-deposited film structure and morphology.
Various Ion Assisted Deposition (IAD) methods have been developed to improve the quality of deposited thin film properties at low substrate temperatures. The deposition source may be an evaporation source (thermal or electron-beam), magnetron sputtering, and the ion assist provided by an ion source, such as, for example, a Kaufman-type gridded ion source or a gridless ion source, such as an End Hall source.
IAD processes are useful for improving properties of films deposited on flat substrates because energetic ions stimulate and cause atomic displacement at the surface, as well as surface atom diffusion and desorption at low substrate temperatures. Control of the incidence angle of the ions and flux of the ions relative to that of the depositing neutral particles may be useful to affect film structure (in particular to increase the film density and/or modify film stress). The ion energies used for ion bombardment in the conventional IAD process are typically at or near the sputtering threshold of the material on the surface and the ion flux is relatively low compared to the deposition flux.
Another known ion assisted method is Dual Ion Beam Deposition, in which a primary deposition ion beam source sputters material from a deposition target to the substrate and a secondary “assist” ion beam source is directed to the surface of the substrate. This method, like other ion assist methods, has the advantage that the angle of incidence of the assist ion beam can be controlled to affect the film properties. Yet another type of IAD method used for plasma-based thin film deposition processes such as sputtering is biased substrate deposition. In this method, ions in the plasma are directed to the substrate by an electric field. However, in this method the ion bombardment of the substrate occurs at essentially normal incidence. In experiments undertaken by the present inventors, Aluminum Oxide films formed by this method tend to form seams at the edges of the step features, where material deposited on the step feature at a first relative angle meets material deposited in surrounding areas at a different relative angle. The resulting seam defect is seen in the micrograph of FIG. 1A. The low quality of material adjacent to these seams is particularly evident when wet etching is used to remove poor quality deposited material, which preferentially etches the material found along the seams leaving voids as are visible in FIG. 1B. These examples represent a typical situation, as deposition of thin films commonly is performed on three dimensional surfaces. Three dimensional surfaces are often involved at some stage of fabrication for most devices, for example, as a result of an accumulation of multiple steps of patterned deposition and etching. Variation in deposited thickness over substrate features can result in problems due to poor conformal coverage, build-up of surface irregularities, trapped voids, seams, and similar problems in the corners of the features. A conformal film is one that has a thickness that is the same everywhere. Variations in device dimensions and properties become more critical as device dimensions are scaled down in size.
It is generally appreciated that the deposited film properties such as density, stress, and optical indexes are dependent on deposition incidence angle. Poor film properties seem to be associated with higher incidence angles. The quality and conformality of films deposited on 3-D surfaces may thus be improved to some extent by controlling the angle of deposition on the substrate (tilting the substrate relative to direction of flux). In a tilted deposition process the substrate is typically also rotated in order to obtain uniform deposition around the 3-D features across the substrate surface. This technique is used in thin film evaporation and ion beam deposition systems, and has more recently has been extended to sputtering systems with the popularization of low pressure sputtering technology. Desirable properties of the film deposited on the bottom and sidewall features of a 3-D feature have been observed for incidence angles of up to, but not exceeding, a critical angle of 55-65 degrees for either bottom and sidewall surfaces. However, control of incidence angles can be achieved only at very beginning stage of the growth. During growth, the shape of the sidewall evolves, and eventually results in glancing deposition angle on the bottom as well as on sidewall surfaces. As a result, quality of deposited material in the corner may deteriorate.
In one known example, thin films are deposited using magnetron sputtering, with the sputtering source at a 45 degree angle to the substrate, and with the substrate rotating to accomplish even coating across the surface. This approach does improve the quality of sidewall coverage on three dimensional features because the sidewalls are deposited with material at an incidence angle nearer to normal. However, experiments conducted by the present inventors have revealed that even an angled deposition process of this kind eventually forms seam lines between field and feature deposition due to the evolution of the sidewall shape described above, albeit less pronounced than those formed in the process described with reference to FIGS. 1A and 1B. These seam lines can be seen in FIG. 2A. Subsequent evaluation by wet etching that is preferential to the lower quality material leaves voids along these seam lines at the periphery of the underlying three dimensional features as seen in FIG. 2B.
Another known ion etch assisted deposition method uses a dual ion beam approach, in which a beam with ion energies well above the material's sputtering threshold is directed to the surface during material deposition. This approach can be used to improve conformality of films deposited on 3-D surfaces. Specifically, in Improved step coverage by ion beam resputtering, J. Vac. Sci. Technol. 18(2), Harper, J. M. E., G. R. Proto, and P. D Hoh (March 1981) (the “Harper paper”), SiO2 films were deposited by an IAD method on a Nb substrate having approximately 90 degree steps, using a dual ion beam deposition system (IBD) in which the angle of incidence of the depositing neutral particles was 20 degrees from the substrate normal and the angle of incidence of the ions from the “assist” (etch) source was 20 degrees from the substrate normal or 40 degrees from the direction of the depositing angle. The general configuration of the system is seen in FIG. 3A. According to the Harper paper, the step coverage was improved, however, the methods in the Harper paper failed to achieve a satisfactorily conformal film. Films deposited according to these prior art methods show a re-entrant overhang of the coating 9 at features in the substrate 8, as seen in FIG. 3B, and/or tend to form a thin facet on the corner of the step, as seen in FIGS. 3C and 3D. Thus, there remains a need to improve upon this and the other known methods for ion etch assisted deposition.
Known methods have thus failed to provide films of desired quality, including films on surfaces with 3-D topology. Particularly, known methods generally result in films having incomplete conformality and uniformity of coverage over the substrate. Thus, a need exists in the art for improvements relating to thin film deposition methods.